can: mark bittiming_const pointer in struct can_priv as const

This patch marks the bittiming_const pointer as in the struct can_pric as
"const". This allows us to mark the struct can_bittiming_const in the CAN
drivers as "const", too.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
diff --git a/drivers/net/can/usb/ems_usb.c b/drivers/net/can/usb/ems_usb.c
index 7ae65fc..086fa32 100644
--- a/drivers/net/can/usb/ems_usb.c
+++ b/drivers/net/can/usb/ems_usb.c
@@ -889,7 +889,7 @@
 	.ndo_start_xmit = ems_usb_start_xmit,
 };
 
-static struct can_bittiming_const ems_usb_bittiming_const = {
+static const struct can_bittiming_const ems_usb_bittiming_const = {
 	.name = "ems_usb",
 	.tseg1_min = 1,
 	.tseg1_max = 16,